In these days, H.261 and H.263, recommended by ITU-T (International Telecommunication Union Telecommunication Standardization Sector), and MPEG-4 (Moving Picture Experts Group Phase 4), internationally standardized by ISO/IEC (International Organization for Standardization/International Electrotechnical Commission), have been known as moving picture compression encoding systems for efficiently transmitting a moving picture signal at a low bit rate. Also, H.264/MPEG-4 AVC (Advanced Video Coding), internationally standardized by the ITU-T and ISO/IEC, is attracting notice as being a system capable of transmitting a moving picture signal more efficiently than the above mentioned moving picture compression encoding systems.
The moving image compression encoding system uses intra-prediction that executes encoding using only image data of the current frame and inter-prediction that executes encoding using images of a past frame and/or a future frame.
With the intra-prediction, DCT (Discrete Cosine Transform) is applied in encoding to an input image frame, in terms of a macro-block as a unit, to generate DCT coefficients, which DCT coefficients are then encoded by variable length encoding. Or, prediction is carried out on a per macro-block basis or on a per block basis obtained on further subdividing the macro-block, using pixels of near-by blocks, such as left or upper blocks: DCT or DIT (Discrete Integer Transform) is then applied to a prediction residual signal to generate DCT or DIT coefficients, which are then encoded by variable length encoding.
With the inter-prediction, in encoding an input image frame, motion compensation prediction is executed with decoded pixels of past and/or future frames to find a prediction residual signal. DCT or DIT is then applied to the residual signal, after which the motion vector as well as DCT coefficients are encoded by variable length encoding.
In these days, such a system that interconnects terminals of different sorts, such as mobile phones or PCs, or terminals of the same sort but different in functions, is being discussed.
In such interconnection, there is a difference in capabilities of terminals, depending upon the sorts of the terminals, such as mobile phones or PCs, or upon functions of the terminals. Thus, data communication which is adapted to different terminals is required.
In data communication of moving images, image size that may be displayed may differ from one terminal to another because of difference in limitations imposed by respective terminals. It is thus necessary to provide a conversion apparatus that converts an encoded bit stream of a moving image (encoded data) to one which fits in with a terminal of interest.
However, to execute size conversion of received encoded data, it is necessary to decode the received encoded data, to convert the image size and finally to re-encode the moving image signal having image size changed.
Moreover, in case a network of destination of connection differs in bandwidth, in particular, is narrower in bandwidth, it is necessary to suppress the bit rate of the encoded data. However, if simply the bit rate is suppressed, the image quality is degraded. It is thus necessary to exercise caution to maintain the code amount per block in each frame such as by contracting the image size or by decreasing the frame rate.
In Patent Document 1 (JP Patent Kokai Publication No. JP-P2002-142222A), there is disclosed an image size changing device in which an image compression-encoded along a spatial axis and along a time axis may speedily be converted into image encoded with different resolution. A variable length decoding means decodes a compressed image to output an orthogonally-transformed image and motion vector information for each frame. The orthogonally-transformed image is then subjected to inverse quantization. A layered inverse orthogonal transform means executes inverse orthogonal transform to the orthogonally-transformed image in accordance with a change rate of the number of pixels to output a non-compressed difference image or a non-compressed changed in resolution. A motion vector correction means corrects the decoded motion vector information in accordance with a change rate of the number of pixels. A motion vector generation means generates motion vector information after the resolution change using the decoded motion vector information. A motion compensation non-compressed image generation means then uses the motion vector information after the resolution change to execute the processing of motion compensation of the non-compressed difference image to generate a non-compressed image. A motion compensation non-compressed difference image generation means uses the motion vector information after the resolution change to generate a non-compressed difference image or the non-compressed image. The non-compressed difference image or the non-compressed image is then compressed on encoding. The non-compressed image following the resolution change may directly be generated from the compressed image previous to the resolution change.
In Patent Document 2, there is disclosed an encoded data image size conversion apparatus whereby the image size of data encoded using prediction of motion compensation and orthogonal transform may readily be changed with improved image quality. With the encoded data image size conversion apparatus, macro-block image data, converted from input encoded image data to a preset image size, may be obtained by an image data conversion unit. A motion vector of the encoded image data supplied is multiplied by a motion vector conversion unit with a preset conversion ratio to obtain a converted motion vector for the macro-block image data. In case a conversion ratio is less than 1, an integrated motion vector is found depending upon the variance of a plurality of converted motion vectors necessary for conversion of the image data. Encoded data of a size-changed image is obtained using the integrated motion vector and the macro-block image data as found.
In Patent Document 3, there is disclosed a digital moving image decoding apparatus wherein an output image size may be changed by a simplified configuration. The digital moving image decoding apparatus is not susceptible to image quality degradation even in an image portion which moves intensely. Moreover, it is unnecessary to perform the processing of low pass filtering or decimation following the processing of decoding. The decoding apparatus includes a motion vector conversion unit that converts the value of the motion vector from the motion vector information included in a bit stream. In case the magnitude of the motion vector is small, inverse DCT is performed using only DCT coefficients of a lower frequency side and, in case the magnitude of the motion vector is large, inverse DCT is performed using not only DCT coefficients of the lower frequency side but also DCT coefficients of a high frequency side.
In Patent Document 4, there is disclosed an encoding apparatus that executes re-encoding in accordance with an encoding scheme which is a combination of motion compensation prediction and DCT. With the encoding apparatus, the calculations amount to be performed when encoded second video data with the changed image size is to be obtained from encoded first video data may be reduced on the whole. In decoding the encoded input data, necessary for re-encoding, the image size is converted in the DCT domain in connection with prediction error. It is then necessary to perform IDCT (Inverse DCT) processing followed by decoding into image data by motion compensation prediction in the spatial domain. The volume of calculations in IDCT processing and image size conversion may thus be decreased to prevent the volume of calculations in motion compensation prediction decoding in the DCT domain from increasing.                Patent Document 1: JP Patent Kokai Publication No. JP-P2002-142222A        Patent Document 2: JP Patent Kokai Publication No. JP-P2002-344973A        Patent Document 3: JP Patent Kokai Publication No. JP-P2001-112002A        Patent Document 4: JP Patent Kokai Publication JP-P2002-374536A        